Hard rock mining is typically facilitated by mechanical equipment such as drills and other dedicated machinery, chemical explosives such as TNT, and/or electrical blasting methods using high energy electrical discharges across spark gaps to create a plasma from an arc of current. The chemical and electrical blasting methods produce rapidly expanding gases within a confined area at the end of holes drilled into rock and thus break up the rock. Where practical, electrical blasting methods are generally preferred because they are less volatile than chemical explosives such as TNT and generally safer to use. Furthermore chemical explosive materials are susceptible to unintended detonation through physical changes, electrical apparatus initiate explosions only through coupling electrical energy and are otherwise inert. The use of mechanical equipment is the most inefficient and time consuming technique used in hard rock mining and thus is often used in combination with blasting techniques.
Electrical blasting methods such as exploding wire and spark gap systems are known for producing an explosion or the venting of a propellant gas. Exploding wire propulsion systems are exemplified by U.S. Pat. No. 5,052,272 to Lee entitled "Launching Projectiles with Hydrogen Gas Generated from Aluminum Fuel Powder/Water Reactions" issued Oct. 1, 1991. Lee discloses a method of generating hydrogen gas with high energy efficiency by applying pulse power techniques to a trigger wire or foil and eventually to an aluminum fuel powder-oxidizer mixture. The preferred oxidizer for the aluminum fuel powder is water. The apparatus includes a capacitor bank connected to an induction coil. A metal wire is connected to the induction coil and a fast switch. when the switch is closed, electrical energy from the capacitor bank flows through the inductor and the switch as well as the wire. The total energy of the electrical discharge is preferably from 0.50 to 15 kilojoules per gram of aluminum fuel. The discharge lasts between 10 and 1000 microseconds.
Another related exploding wire blasting system is disclosed in U.S. Pat. No. 3,583,766 to Padberg, Jr., entitled "Apparatus For Facilitating The Extraction of Minerals From The Ocean Floor, " issued Jun. 8, 1991. In particular, the '766 patent discloses a deep submergence search vehicle having a drill pipe into a bore formed in a layer of mineral deposits and extending into a sedimentary ocean bed. A drill head is positioned at the lower end of the drill pipe with a plasma discharge section positioned above the drill head. An energizing circuit couples the electrical energy from a power source to a thin nickel wire extending through the plasma discharge section. When a switch is closed, a high current is suddenly passed through the thin nickel wire exploding it and creating a large plasma discharge accompanied by sharp pressure waves. Openings in the plasma discharge section allow the pressure waves to emerge and produce a rapidly expanding and collapsing gas bubble with accompanying shock waves simulating those of explosives. The bubble expansion and collapse propagates acoustic waves in the form of sharp pressure pulses.
Still another related art exploding wire blasting system is disclosed in Soviet Union No. SU357345A to Yutkin which shows a rock breaking device having a pair of electrodes and a conductive wire strip for insertion in a hole in rock filled with a wetted dielectric bulk material, such as sand, to produce shock waves when energized. The wire is connected to the electrodes and stretched around a dielectric plate. The dielectric plate is positioned in the rock hole for bursting operation.
Spark gap or non-exploding wire systems are exemplified by U.S. Pat. No. 3,679,007 to O'Hare, entitled "Shock Plasma Earth Drill," issued Jul. 25, 1972, which disclose a spark gap probe for drilling deep holes in the earth for the recovery of water or oil. The probe has a center electrode separated from and surrounded by an outer electrode both of which are immersed in water. A condenser or capacitor bank is charged to a potential of 6000 to 30000 volts (depending upon soil conditions) which supplies electrical energy to the electrodes. Rapid release of electrical energy across the resistance of the water produces a large amount of heat to produce an explosive effect. The explosive shock waves generated in the water move downward and outward to produce a hole into which the earth drill repeatedly falls.
U.S. Pat. No. 4,741,405 to Moeny et al., entitled "Focused Shock Spark Discharge Drill Using Multiple Electrodes," issued May 3, 1988, discloses a spark gap discharge drill for subterranean mining. The drill delivers pulses of energy ranging from several kilojoules up to 100 kilojoules or more to a rock face at the rate of 1 to 10 pulses per second or more. A drilling fluid such as mud or water assists propagation of the spark energy to the rock face.
U.S. Pat. No. 5,106,164 to Kitzinger et al., entitled "Plasma Blasting Method," issued April 21, 1992, discloses a plasma blasting process for fragmenting rock in the practice of hard rock mining and more particularly teaches a method which uses rapid and very high energy discharges across electrodes in an electrolyte. The electrical energy from a capacitor bank is switched to supply 500 kiloamperes to a blasting electrode positioned within a bore in a rock face causing dielectric breakdown of an electrolyte, preferably containing copper sulfate. The electrolyte may be gelled with bentonite or gelatin to make it viscous enough so that it does not leak out of the confined area prior to blasting. The blasting apparatus has minimal inductance and resistance in order to reduce power loss and ensure rapid discharge of energy into the rock.
Whereas the electrical blasting methods taught thus far us simple electric spark gaps and exploding wires to generate a very large electrical discharge from charge stored in capacitors delivering hundreds of kiloamperes of current and may involve the use of electrolytes, it would be desirable to develop a blasting method operable at more moderate energy levels. Additionally, most of the prior art high voltage electrical methods transfer the energy from the capacitor bank to the explodable conductor or spark gap in a relatively inefficient manner. As a result of the inefficient transfer of energy, the related art systems need relatively large capacitor banks for driving either the explodable conductor or the spark gap to provide a given amount of explosive energy.
Alternatively, many blasting systems that utilize chemical explosives present significant safety concerns due to the sensitive nature of common explosive materials. Many explosive materials are susceptible to unintended detonation through physical impact, stray electric charges, and severe environmental conditions (i.e. high temperatures). In addition, many blasting techniques that utilize chemical explosive materials can produce toxic by-products and often pulverize surrounding rock material, which can be undesirable in some applications. Thus, it may be desirable to develop an approach to breaking rock in which highly insensitive and non-toxic explosives are utilized which require only moderate energy electrical initiation or ignition as is used in electrical blasting systems. Such a combination would present a safe, economical and efficient blasting technique that is somewhat more gentle fracturing process than is offered with a high explosive charge.
It would also be desirable to combine relatively safe chemical blasting methods and/or electrical blasting methods with mechanical drills thereby speeding up the drilling/blasting process and facilitating its automation. Many hard rock mining operations typically involve both drilling and blasting operations, that if properly combined or integrated would eliminate the need to withdraw the mechanical equipment from the bore hole and insert a separate blasting probe or explosive charge. Several of the aforementioned related art attempt to combine the drilling and blasting processes within a single piece of equipment. See e.g. U.S. Pat. No. 3,679,007 to O'Hare; U.S. Pat. No. 4,741,405 to Moeny et al.; and U.S. Pat. No. 3,583,766 to Padberg Jr. Due primarily to the destructive nature of many chemical blasting techniques, none of these related art systems have successfully combined chemical blasting techniques with the mechanical drills.